So I hooked up the PM660 into my main system and poop, it's sounds like crap. Now I know what grainy means. Compared to the 3W/ch single-ended cathode follower tube amp I've been using, it sounded horrible. It gets loud, but the sound is unbearable. I ordered most of the caps for the driver and power stage and have some homework to do.
The PM-660 is an excellent amplifier, when it is working. Unfortunately, the driver board is a known problem area. It uses a vertically mounted sub board and dry joints are a common problem. As it uses no protection (no relays, just muting), you need to check all the connections are good. Trust me, you don't want it to give you trouble.
Do not start poking around with capacitors- you are wasting your time and will likely cause problems you don't have right now. There are absolutely no capacitors in the signal line from input to output when main direct mode (-7dB) is engaged and the amplifier is ruler flat from 0.5Hz-well over 120KHz. Before you do anything. At all. Test the amplifier. What you are describing is either a) your imagination or b) the other amplifier is non-linear to the point it is audible.
The biggest issue with all of the HKs from that era are the silver plated pushbutton switches for everything. They seriously oxidise inside and there is only one way to fix them. Do not use any contact cleaner. They must be removed from the PCBs and dismantled and cleaned internally with silver polish, then ultrasonically cleaned, dried and re-assembled. Not a job for a beginner. The issues will be most evident in the MM/MC headamp switches.
Ask me anything about the PM-660, I have 3 of them, 2 PM-650s and a few PM-640s. I love them, with the PM-650 (the model below the 660) being my favourite. In fact there's two in my loungeroom right now to play with as they have been in storage for about 15 years.